Resumo em Inglês:

Abstract A comparative analysis of three-dimensional (3D) modifications of Hoek & Brown (1988) rock failure criterion was carried out. The correlations between failure stress and the other principal stresses were first determined using polyaxial test data for five geomaterials including KTB amphibolites, Westerly granite, Dunham dolomite, Shirahama sandstone and Yuubari shale. The prediction accuracies of five non-recursive, three-dimensional modifications to Hoek-Brown failure criteria and the original two-dimensional Hoek-Brown criterion were later investigated using root mean square error and coefficient of determination as measures of misfit. The results reveal that the intermediate principal stress significantly affects strength in geomaterials like the Dunham dolomite. It also moderately affects the strength of geomaterials like the KTB amphibolites, the Westerly granite and the Yuubari shale. Moreover, the intermediate principal stress has mixed effects on strength in the Shirahama sandstone. In addition, the original Hoek-Brown failure criterion could still be used with reasonable accuracy for geomaterials whose strength shows low dependence on the intermediate principal stress. While a three-dimensional Hoek-Brown criterion must be used for geomaterials like the Dunham dolomite, whose strength shows a high dependence on the intermediate principal stress. The original Hoek-Brown failure criteria should be used with caution for geomaterials like the Shirahama sandstone, the KTB amphibolites, the Westerly granite, and the Yuubari shale, whose strength shows either mixed or intermediate dependence on the intermediate principal stress. Average prediction accuracies followed the order: simplified Priest (2012), Ma et al. (2020), and Jiang & Zhao (2015). Both original Hoek & Brown (1988) and Li et al. (2021) criteria were tied, while Liu et al. (2019) was the least.

Resumo em Inglês:

Abstract The paper presents a comparative study between semi-empirical methods for the estimation of pre-consolidation pressure and undrained shear strength from Piezocone (CPTu) data. The first method, proposed by Massad, was developed from observing the variation of these parameters with depth; the second method, proposed by Mayne, was developed from simplifications and relationships between the Spherical Cavity Expansion Theory (SCET) and the Critical State Theory; the third method was proposed by Mayne, which considers the variations due to soil type from the CPT Index to estimate the pre-consolidation pressure. The methods were validated based on their applications to the marine clay from Santos Coastal Plain, Brazil, Bothkennar clay from Scotland, and Torp Clay from Sweden. It is intended to verify if the results are consistent with each other, with the stress history of these soils and with the available test results. The application of the Massad’s method led to results close to the available reference values. The results of the Mayne’s method based on SCET showed great variability in behavior comparing to the test data depending on the case study. By the Mayne’s method based on CPT Index values, the calculated pre-consolidation pressures were slightly higher than the values of the available test data. The variations in the results highlighted the importance of validating estimates based on semi-empirical methods through specific tests and the knowledge of geological history contributes to predicting the behavior of clays, since they showed good agreement with the available data from oedometer tests.

Resumo em Inglês:

Abstract Due to the importance of surface and subsurface settlements to prevent damages to building foundations and sensitive structures in the urban cities, in this study, the ABAQUS finite element software has used to conduct a series of numerical modeling analysis on ground surface settlement caused from the asynchronous excavation of twin-tunnel. The effects of tunnel diameter, center-to-center tunnel spacing, and tunnel depth are discussed in detail and the shape of the surface settlement curves is also plotted. The numerical modeling has been verified by the results of three sequential twin-tunneling centrifuge tests conducted by the City University of London with 94.22%, 98.71% and 99.56% accuracy, respectively. Based on the results of this study, reducing the tunnel diameter decreases the amount of the maximum ground surface settlements and reducing the depth of tunnels and the distance between twin-tunnel to less than 2D (D is the diameter of the tunnels) increase the maximum surface settlements. Installation of 30 cm of tunnel lining can decrease the maximum ground surface settlement up to almost 79%.

Resumo em Inglês:

Abstract Chemical stabilization for mining tailings is a promising alternative to enable their use as construction materials. For this, it is necessary to evaluate the behavior of these composites to ensure minimum design requirements. This research aims to demonstrate that an addition of 15% of polymeric solution content, corresponding to 6% of polymer by tailings mass unit, can improve considerably the mechanical strength of gold ore mining tailings. To this end, unconfined compression and direct shear tests were conducted, indicating an increase in compressive and shear strength, especially with 28 days of curing time and at maximum dry unit weight. Microstructural and chemical tests were also performed, demonstrating that the tailings have silt-sized particles, mostly composed of quartz, muscovite, and kaolinite. Despite its granulometry, the tailings do not exhibit cohesive behavior and require to be considered perhaps as rock sediments. Scanning Electron Microscope analysis showed that the particles are lamellar, and a more stable arrangement contributes to the polymer's performance as a binder. It was observed that the strength gain occurs due to polymer bond effect and to the matric suction.

Resumo em Inglês:

Abstract Stabilized soil with cement has been a widely used solution in geotechnical projects and requires the molding of numerous specimens to investigate their properties in destructive tests, restraining the reuse for other analysis. Also, distinct sample dimension has been utilized in the research without taking this effect into account. Recognizing these needs, the height and diameter (H/D) ratio effect on cement-stabilized soil mechanical behavior was assessed in the present work. Using sandy soil, Unconfined Compression Strength (UCS), Indirect Tensile Strength (ITS) and Ultrasonic Pulse Velocity tests were performed varying the curing period (7 and 28 days), cement type (I and III) and content (6, 7, 8, 9 and 10%), based on conventional (127 x 100 mm) and reduced dimension (105 x 50 mm) specimens. All variables, individually and combined, affected the compression and tensile strengths of the mixtures, resulting in greater gains for the ones molded with type III cement in reduced dimension, at 28 days of curing. The dimension as an influential variable was statistically established using the variance analysis (ANOVA) at a significance level (α) of 0.05, in which reduced dimension showed an average superior resistance of 21.3%. Mixtures molded with H/D ≥ 2 demonstrated strong correlation (R2 = 0.93), pointing to the possibility of ITS prevision through the P-wave velocity nondestructive tests.

Resumo em Inglês:

Abstract The presence of unsaturated flow in tropical lateritic soils of unknown hydromechanical behavior may lead to severe geotechnical problems. It is known that soil-water characteristic curves (SWCC) are a valuable tool to define this behavior and facilitate the suction estimation, acting on tests performed with no control of such parameter. This article aimed to determine and model the SWCC for 3 unprecedented tropical soils, in the undisturbed and the compacted conditions, from the Paraná state, located in the south of Brazil. The soils studied are one clay and two sands, all lateritic. The pressure plate and the filter paper methods were used to determine the SWCC. The samples were carved and submitted to the drying paths. Furthermore, Mercury Intrusion Porosimetry (MIP) tests were performed in samples of the studied soils in order to obtain the pore size distribution (PSD), contributing to the bimodal definition of curves and the prediction of SWCC main parameters. Both techniques (pressure plate and filter paper) were combined and the SWCC was adjusted by Gitirana & Fredlund model, which efficiently represents the shape of the traditional curves for tropical soils. Furthermore, the SWCCs were predicted by PSD and obtained satisfactory numerical fits. All SWCC presented air entry values in the macropores that are characteristic of soils as the lateritic, given that its structure was considered a key factor in the shape of the curves.

Resumo em Inglês:

Abstract For the purpose of road safety, it is vital to reduce the severity of road accidents and increase safety around the roadway area by deploying guardrails. In case of a car crash, a guardrail post must be deformable so that such restraint is not too abrupt due to the occupant’s sensitivity. Soil type influences on the guardrail post behavior have been a somewhat unfounded variable due to the high soil heterogeneity and challenging interpretation of its real implications on the safety of guardrail systems. Since little attention is concentrated on evaluating the guardrail post behavior through simplified procedures, this article aims to provide a simplified experimental and numerical approach to study the behavior of guardrail posts located on organic soil. Results of laboratory and in-situ tests indicated that guardrail posts behavior located on organic soil depends on section orientation, driving depth, and loading speed. To confirm and compare the in-situ tests, simplified numerical simulations through Plaxis 3D software were carried out, and data from numerical modeling approved the accuracy of in-situ results.

Resumo em Inglês:

Abstract Dam B1 was approximately 85 m high and 700 m long along the crown when it suddenly failed in 25th of January, after 41 years of operations and deposition of tailings at the Paraopeba mining complex (Côrrego de Feijão), in Minas Gerais, Brazil. More than 250 people died and vast economic, social and environmental damages resulted from the collapse. Given the need to geotechnically characterise the tailings within the dam for a credible computational model of the failure, an extensive sampling and laboratory testing campaign took place. The geotechnical laboratory of the Civil Engineering Department at the Faculty of Engineering of the University of Porto (Portugal) was invited by CIMNE, under a contract with VALE, with MPF agreement, to conduct this program. This paper will present a description of the sampling campaigns in the remaining post-mortem dam/reservoir tailings were the failure instability mechanics developed and the experimental program undertaken to test the tailings in advanced laboratory tests. The results that embrace evaluations of the physical, hydraulic and mechanical properties, deduced from integral samples selectively collected and prepared as described, which allowed to define the geotechnical parameters necessary for the referred analyses are here resumed.

Resumo em Inglês:

Abstract Understanding the geotechnical properties of iron ore tailings is currently one of the major challenges in the mining industry. With transitions from drained to undrained conditions occurring in seconds, recent dam problems have been a challenge to solve with classical soil mechanics, which provides few means to explain how such phenomena develop. There is also an increasing propensity in technical and scientific circles to seek constitutive models that are based on critical state soil mechanics and that allow for the analysis of tailings behaviour. However, there is still a lack of knowledge and information about the critical state properties of iron ore tailings. The present research experimentally and numerically investigated the effectiveness of modelling the behaviour of iron ore tailings. The aim of these experiments was to assess the critical state parameters of tailings from a significant iron ore operation site in Quadrilátero Ferrífero (Minas Gerais state, Brazil). The results indicated that the selected numerical model (NorSand) was adequate to evaluate the behaviour of the studied mine tailings. The numerical results showed consistent adherence to the experimental results of both drained and undrained tests, with deformations below 5% and samples in which the state parameter had a small magnitude.

Resumo em Inglês:

Abstract Soil mechanical fully understanding requires considering a three dimensional approach, including soil response under the second principal stress and its potential anisotropy. In order to achieve such soil mechanical understanding, a true triaxial apparatus might be used. Therefore, in the present research an automated true triaxial apparatus was developed comprising its cubical cell, data acquisition and stress control systems. The manufactured apparatus was validated by means a laboratory test campaign where true triaxial test responses were compared to standard drained triaxial tests. True triaxial and standard drained triaxial tests were carried out on both naturally and artificially cemented soils. Results were gathered and compared. A soil mechanical compatibility was observed when test results of the newest true triaxial equipment were compared to test results obtained from a well validated standard triaxial apparatus. Thereby, the present paper reports an affordable successful true triaxial apparatus development demonstrating its efficiency for regular soil mechanical tests. Finally, a full stress rosette was established for a uniform fine sand where some small anisotropy was detected.

Resumo em Inglês:

Abstract Mining is an important economic activity in the modern world. However, despite the generated benefits, mining produces tremendous volumes of tailings, an environmental liability with numerous adverse effects. Researches about contaminant transport in tailings dam are important to assess the degree of contamination and to propose preventive or remedial measures. In geotechnical practice, the flow of solutes is generally characterized by numerical solution of the Richards equation to describe water movement followed by advection-dispersion equation to describe contaminant movement. This study aimed to model and simulate contaminant transport in a laboratory column test, using a new analytical formulation and mathematical codes, through tailings in transient unsaturated conditions. The analytical solution for the Richards equation was used to simulate the variation in the volumetric water content and to determine the transient contaminant plume using the advection-dispersion equation subsequently. The models were used to calibrate experimental data from hydraulic characterization and contamination tests. Finally, the normalized contaminant plume (cw/c0) was simulated as a function of time and space. Comparisons with experimental data showed that the analytical formulations adequately expressed the process of contaminant infiltration through the unsaturated porous medium. The formulations offered effectively and are configured as a new approach to solve various contamination problems in transient unsaturated conditions, providing insights into many complex processes that occur in the lab tests and requires far less computational effort compared with current programs to modeling the solute transport using numerical solutions, as the versatile commercial Software HYDRUS.

Resumo em Inglês:

Abstract Allying technology, theory and engineering practice is one of the main challenges of modern foundation engineering. Current foundation and retaining walls designs may lead to oversizing or undersizing due to the spatial variability of the stratigraphic profile. As the design procedure of piled foundations involves defining the setting depth of the tip, sometimes the piles may not behave as expected, as the soil mass may change significantly even in a small region. In this paper, a study was conducted on the geoenergetic behavior of the soil mass and of the piles of a construction site located in Brasília, DF, Brazil. Previous studies showed that the energy needed to drill a continuous flight auger pile (CFAP) can be related to the strength of the drilled strata. Therefore, a methodological framework was built to further discuss how the geomechanical behavior of CFAPs foundations could be assessed by analyzing the energy needed to drill such piles. Statistical methods, in special the Bootstrap method, were used to assess the possible influences of construction procedures on pile behavior. It was studied how the execution energies of piles vary with initial depth and relative position in the terrain, as well as the influences that they suffer because of the order of execution within a same foundation group and due to the proximity to surrounding retaining walls. The proposed methodology can be used to improve the energy control procedure and performance evaluation of CFAPs, allowing a complementary reliability assessment for the foundation and retaining walls designs and implementations.

Resumo em Inglês:

Abstract The test sites analyzed here consist of clay deposits located in the Jacarepaguá Lowlands in Rio de Janeiro, characterized by high plasticity, high compressibility and low undrained shear strength. The deposits are made up of lightly overconsolidated aged clays, montmorillonite being the predominant clay mineral. Soft clay deposits are usually superficial, with thicknesses generally varying between 6 m and 17 m and geologically recent and originated from marine regressions and transgressions, that occurred between 6000 and 3500 years ago. The objective of this study is to analyze a large database of undrained shear strength measurements obtained by 461 vane tests performed at 15 different sites. In general, most of the data correspond to very soft clays, with undrained shear strength values lower than 25 kPa. The undrained shear strength measurements are correlated with plasticity index and with maximum excess pore pressure, measured with piezocone tests. The method for estimating the undrained shear strength su(DT) of soil from the excess pore pressure generated during piezocone dissipation tests proposed by Mantaras et al. (2015) was validated against the vane test database.

Resumo em Inglês:

Abstract Butadiene-styrene copolymer (SBR) is an elastomer composed of 75% butadiene and 25% styrene and is widely used in the automotive industry in tire production. This elastomer can be produced from two polymerization processes: emulsion or solution polymerization. This paper presents the mechanical behavior of a polymer reinforced sand compared to pure sand. Direct shear tests were performed on pure sand specimens and with the addition of modified styrene butadiene-styrene copolymer (XSBR). The polymeric sand specimens had 10% moisture content, 50% relative density, with water-polymer mass ratios of 1:1, 1:2, and 1:4, with no curing time, or with curing times 48, 72, 96, 576, and 720 h. Improvements were verified in the strength parameters of sand specimens with polymer addition, while comparing with pure sand parameters, showing that the improvement of soils with polymers is satisfactory for application in geotechnical works, such as: embankments in soft soils, soils for shallow foundations and for slope stability.

Resumo em Inglês:

Abstract This paper deals with engineering geological properties of Qadis Khordak dam site in Northwest Afghanistan. This study is based on on-site and laboratory tests, surface discontinuity surveying, drilled borehole and permeability test of dam foundation. The engineering geological properties at the dam site were studied in order to evaluate geotechnical characteristics of rock masses at dam foundation, geotechnical properties of alluvium at dam axis, reservoir and borrow materials. The structural geological studies also carried out due to stability and safety of dam on their abutments reservoir and seismicity. Existence of a fault, high permeable zone at dam foundation and the thickness of alluvium at dam axis, are the most engineering geological issues that cause change on dam type selection. In the feasibility phase, the dam type was chosen as the concrete face rock-fill dam, because of state of engineering geological properties of dam site. However, in the design phase dam type has been changed as a rock-fill with a clay core.